1. Field of the Invention
The present invention generally relates to computer chip manufacture, and more particularly to etch techniques used in constructing integrated circuit structures.
2. Background Description
In the early days of production of integrated circuits, patterned resist masks were used in concert with liquid, aqueous etches to remove the underlying, regions that were not masked with resist. There was deterioration of the lithographically defined dimension caused by undercut of the resist masks by the isotropic character of the liquid etch. For this reason, more expensive reactive ion etching has largely supplanted the early method of liquid etching. However, vertical surfaces cannot easily be patterned with reactive ion etching.
Sometimes it is necessary to mask liquid phase etching of silicon dioxide or doped silicon dioxide. The most convenient way is to use resist to define the region to be etched. However, aqueous solutions of hydrogen fluoride can attack the interface between the resist and oxide leading to undercut of the resist with the best process conditions and leading to complete removal and failure of the resist mask under the worst conditions, where capillary action draws the aqueous solution completely under the resist. If the resist detaches, then all the wafers can be contaminated by resist particles. Etching with conventional, gaseous vapor hydrogen fluoride is also known to penetrate resist, and can even lead to enhanced etching under the resist. The aqueous films formed on the surface in this type of gaseous etching are also subject to capillary action at the resist/oxide interface.
Other well known problems associated with aqueous processing when hydrophobic resist or silicon surfaces are exposed is the deposition of particles that are suspended in solutions and the residue left by drying of droplets of solution. Both problems are associated with hydrophobic surfaces that cause solutions to "bead up" and particles to be attracted. Also, the surface tension of the liquid can prevent penetration and etching of small masked features.
It is possible in many applications to use a gaseous reactive ion etching process to remove unmasked material. This process does not have the resist adhesion, resist undercut, solution penetration, particle contamination from solution, and dried contaminant problems associated with aqueous processing. However, reactive ion etch (RIE) is expensive because only one wafer at a time is processed, the substrates are susceptible to unwanted damage, and tooling is complex. In addition, only horizontal surfaces which are parallel to the surface of the wafer can be patterned because the incident ions which are used to produce etching travel perpendicular to the wafer surface. Masked surfaces perpendicular to the wafer surface are not patterned. There are still applications where a vertical surface must be patterned, or where low cost is a requirement. A new method that does not degrade lithographic dimensions and which can process several wafers simultaneously with low cost would be advantageous.
A particularly important application of patterning of vertical oxide surfaces is in formation of a deep trench for use as in capacitor in Dynamic Random Access Memories. Particles or watermarks left behind on the hydrophobic sidewalls of the "aqueous processed" trench can cause failure of the trench. It is not possible to use Reactive Ion Etching to pattern the vertical trench surfaces.
Vapor phase hydrogen fluoride exposures have used photoresist masks to pattern silicon dioxide. However, conventional anhydrous vapor hydrogen fluoride, hydrogen fluoride/H.sub.2 O and hydrogen fluoride/alcohol reactions act through an intermediary film of liquid. Thus, they are also susceptible to loss of resist adhesion and loss of pattern integrity through capillary action. In addition, the conventional processes take place with relatively high hydrogen fluoride partial pressures of tens of Torr or more. At these relatively high pressures, the hydrogen fluoride can even penetrate the resist layers and preferentially etch the oxide under the masked areas. See, for example, Whidden et al., J. Electro Chem. Soc. 142, 1199 (1995). Furthermore, both the aqueous HF and standard vapor HF process isotropically etch the unmasked region of oxide. It is not possible to etch an oxide layer to produce an exposed region of the underlayer with dimension smaller than the resist opening, or with a mouth opening less than the resist opening and two times the oxide layer thickness.